Solid-state lasers pumped by high-power laser diodes have been used for many years in a wide range of applications. Conventional diode-pumped solid state lasers can be classified as end-pumped and side-pumped lasers, as shown in FIGS. 7A and B.
The end-pumped configuration, depicted in FIG. 7A, usually leads to higher laser efficiency because of a better overlap between the pump beam and the laser cavity mode. At high average power levels, end pumping becomes less attractive because of the difficulty to couple a large amount of laser-diode output into the end of a laser rod, which typically has a diameter of 2-6 mm.
A side-pumped geometry, depicted in FIG. 7B, is preferred for high-power designs because of the large rod surface that can be used for radiation pumping. Conventional side-pump schemes have the disadvantage of short optical path across the diameter of the laser rod, with the consequence that a large fraction of the pump radiation is not deposited into the laser rod. This characteristic also makes side-pump lasers more sensitive to the diode radiation wavelength, which may change during the course of its lifetime. These disadvantages considerably reduce the side-pumped laser efficiency.
A problem plaguing both end-pumped and side-pumped geometries is that of inhomogeneous pumping. The side-pumped laser rod has to balance Beer's Law absorption, that tends to deposit the power near the surface closest to the emitter, against the loss of absorption efficiency encountered when a lightly doped rod is used to permit deeper penetration. In addition, diode light concentration using conventional optics such as lenses often limits the number of emitters that can be circled around the rod and requires careful design to achieve pumping homogeneity.
The end-pumped laser, using an imaging device to demagnify the emitting diode array, results in imprints of the diode array image at the end of the rod. Inside guiding curved surfaces of the rod forms a repeating sequence of these images.
Inevitably, both geometries end up with hot and cold spots. This limits the average output attainable because of wavefront distortion and depolarization.